The contemporary work and home offices use a multitude of paper products including, but not limited to reprographic paper grades and paperboard, such as writing papers, printing paper, copy paper, and forms paper. Unfortunately, such paper and paperboard products exhibit one or more disadvantages. For example, some of these products have relatively low basis weights or are not sufficiently stiff in bending or durable to sustain a full run through a copy machine. Thus, within the industry there is a constant aim to produce reprographic papers at lower basis weights, but at equal stiffness properties, in order to save raw materials and to be able to increase productivity. Other important properties of reprographic papers are curl, i.e. out-of-plane movement, and hygroexpansivity, i.e. expansion and contraction of the paper with varying relative humidities. A low curl is required during stacking of paper in copier machines and for correct feeding. A low hygroexpansivity is required because curl is a function of the hygroexpansivity, and of the material distribution in the sheet (see e.g. Carlsson, L.: A Study of the Bending Properties of Paper and their Relation to the Layered Structure, Doctoral thesis, Chalmers University of Technology, Department of Polymeric Materials, Gothenburg, Sweden, 1980, ISBN 91-7032-003-9). The hygroexpansivity and curl are also a function of the papermaking process, especially during drying of a fibrous web (see e.g. Handbook of Physical Testing of Paper, 2nd Edition, Vol. 1, Chapter 3, page 115-117, ISBN 0-8247-0498-3 by T. Uesaka: Dimensional Stability and Environmental Effects on Paper Properties). The bending stiffness Sb of paper is a function of the elastic modulus E and the thickness t, such that Sb is proportional to Et3. This means that the most effective means to increase the bending stiffness is by increasing the paper thickness. However, the thickness normally must be retained within specifications. An even more efficient way to increase bending stiffness is to create an I-beam effect, i.e. strong dense outer layers and a lower density core. Mathematical expressions of a three-layered structure show that the I-beam effect creates considerably higher bending stiffness compared to a homogeneous structure if all other parameters are kept constant (see e.g. Handbook of Physical Testing of Paper, 2nd Edition, Vol. 1, Chapter 5, page 233-256, ISBN 0-8247-0498-3 by C. Fellers and L. A. Carlsson: Bending Stiffness, with Special Reference to Paperboard). This knowledge has been reduced to practice in multi-ply paperboard as well as for low basis weight printing papers, such as reprographic papers (see e.g. Häggblom-Alnger, U., 1998, Three-ply office paper, Doctoral thesis, Åbo Akademi University, Turku, Finland, 1998).
Modern size-press units of paper machines produce reprographic paper grades commonly having metered size-presses. These units enable the application of size-press starch (and/or other strengthening components) to other layers of the sheet. This technology has been demonstrated in the published literature (see e.g. Lipponen, J. et al.: Surface Sizing with Starch Solutions at High Solids Contents, 2002 Tappi Metered Size Press Forum, Orlando, Fla., May 1-4, 2002, Tappi Press 2002, ISBN 1-930657-91-9). The authors concluded a significant bending stiffness improvement running the starch solution at the size-press at 18% solids compared to lower solids (8, 12 and 15%).
There are also flooded-nip (also called pond or puddle) size-press units in common use. In this instance the potential for application of starch solutions to the outer layers is not the same as for metered size-press units due to inherent deeper penetration into the sheet in the flooded-nip. However, results in the literature suggest that an increase in starch solids can also cause less penetration with potential for improved bending stiffness (see e.g. Bergh, N.-O.: Surface Treatment on Paper with Starch from the Viewpoint of Production Increase, XXI EUCEPA International Conference, Vol. 2, Conferencias nos. 23 a 43, Torremolinos, Spain, page 547-, 1984). There is, however, room for considerable improvement in bending stiffness over the results reported in the literature and to receive other benefits such as stated above.
Accordingly, there exists a need for improved paper and paperboard products that reduce or eliminate one or more of these disadvantages while being able to produce paperboard and reprographic paper grades at considerably lower basis weights, at higher production rates, and, consequently, at lower manufacturing costs. Such an improvement would benefit from increased bulk of the paper web before the size-press application (n.b. the large influence of paper thickness on bending stiffness) in combination with high solids starch solutions including viscosity modifiers and/or crosslinkers to increase the strength of the size-press coating and to increase hold-out attachment of the surface to the applied layer. Further, it is the object of this invention to provide these benefits within a single-ply paper, thereby eliminating the costs associated with the additional machinery required for paper having multiple cellulosic layers.